This patch can trace current xtime and wall_to_monotonic. Then user canuse ctime() to convert them to wall time which is easier to be understood, especially for flight-recorder which need to get trace event from a kerneldump file.

@@ -568,6 +572,7 @@ void update_wall_time(void) /* check to see if there is a new clocksource to use */ change_clocksource(); update_vsyscall(&xtime, clock);+ trace_gtod_update_xtime(&xtime, &wall_to_monotonic); }